1. Field of the Invention
This invention relates to a soy protein concentrate that has desirable flavor, functional and nutritional properties.
2. Description of the Related Art
The benefits of soy protein are well documented. Cholesterol is a major concern with consumers throughout the industrialized world. It is well known that vegetable products contain no cholesterol. For decades, nutritional studies have indicated that the inclusion of soy protein in the diet actually reduces serum cholesterol levels in people who are at risk. The higher the cholesterol, the more effective soy proteins are in lowering that level.
Soybeans have the highest protein content of all cereals and legumes. In particular, soybeans have about 40% protein, while other legumes have 20–30%, and cereals have about 8–15% protein. Soybeans also contain about 20% oil with the remaining dry matter mostly carbohydrate (35%). On a wet basis (as is), soybeans contain about 35% protein, 17% oil, 31% carbohydrates, and 4.4% ash.
In the soybean, both storage protein and lipid bodies are contained in the usable meat of the soybean (called the cotyledon). The complex carbohydrate (or dietary fiber) is also contained in the cell walls of the cotyledon. The outer layer of cells (called the seed coat) makes up about 8% of the soybean's total weight. The raw, dehulled soybean is, depending on the variety, approximately 18% oil, 15% soluble carbohydrates, 15% insoluble carbohydrates, 14% moisture and ash, and 38% protein.
In processing, soybeans are carefully selected for color and size. The soybeans are then cleaned, conditioned (to make removal of the hull easier) and cracked, dehulled and rolled into flakes. The flakes are subjected to a solvent bath that removes the oil. The solvent is removed and the flakes are dried, creating the defatted soy flakes that are the basis of all soy protein products. Despite the large number of products on the market, there are only three types of soy protein products: flours, concentrates and isolates.
Soy flours are high in oligosaccharides and have a “beany” flavor that may be objectionable to some consumers. The lack of optimized processing makes soy flours highly variable in terms of quality.
Soy flours and grits are still widely produced and are used most often in baked goods, snack foods and pet foods applications where the high flavor profile does not pose a problem. Textured soy flours were an early attempt at simulating or enhancing the texture of meat products. Texturizing does not change the composition of soy flours and reduces the flavor profile only slightly. Their primary applications are inexpensive meat products or pet foods.
The oligosaccharides, raffinose and stachyose, in soy flour potentially cause flatulence as their bacterial fermentation in the colon creates intestinal gas. Suarez reported that ingestion of 34 grams (g) of conventional soy flour (1.3 g raffinose and stachyose) caused no significant increase in flatulence frequency, whereas ingestion of 80 g of conventional soy flour (3.1 g raffinose and stachyose) resulted in a significant increase in flatulence frequency. Surarez, Fabrizis L. et al., Am. J. Clin. Nutr., 69:135–9 (1999).
Soy concentrates have at least 65% protein. A myriad of applications have been developed for soy concentrates and texturized concentrates in processed foods, meat, poultry, fish, cereal and dairy systems. Soy protein concentrates are made by removing soluble carbohydrate material from defatted soy meal. The most common means for carbohydrate removal is aqueous alcohol extraction (60–80% ethanol) or acid leaching (isoelectric pH 4.5). In both aqueous alcohol extraction and acid leaching, however, essentially all of the protein is rendered insoluble. Protein solubility may be recovered in acid leach products by neutralization.
Isolates are produced through standard chemical isolation, drawing the protein out of the defatted flake through solubilization (alkali extraction at pH 7–10) and separation followed by isoelectric precipitation. As a result, isolates are 90% protein on a moisture-free basis. They contain no dietary fiber and are sometimes high in sodium, properties that can limit their application. Their major applications have been in dairy substitution, as in infant formulas and milk replacers.
It is known that a soy protein product having a substantially bland taste and colorless appearance may be produced by filtration using a membrane having a molecular weight cut off (MWCO) of 70,000.
In recent years, researches have been conducted to better understand the role of isoflavones in chronic disease prevention. According to the American Institute for Cancer Research, isoflavones may inhibit enzymes necessary for the growth and the spread of many types of cancer such as breast cancer, prostate cancer and colon cancer. Isoflavones also have shown great promise in preventing osteoporosis and treating menopausal symptoms.
Soybeans contain about 0.5% by weight saponins. Soy saponins have been the subject of investigation since the early 20th century. These compounds consist of a triterpenoid skeleton with various sugar and acetyl moieties. The current consensus is that soyasapogenols A, B and E are true aglycons, while other soyasapogenols are artifacts of hydrolysis conditions. The corresponding glycosides are the so-called ‘group A saponins’, ‘group B saponins’, and ‘group E saponins’, respectively.
Soy saponins have demonstrated anti-mutagenic properties that make them promising agents for cancer prophylaxis. Moreover, group B soy saponins have exhibited pronounced suppressive effects on the replication in vitro of the human immunodeficiency virus (HIV). The chemical structure of soybean saponins is very similar to that of the compound glycyrrhizin, a known anti-viral agent, so soy saponins show promise as building blocks for the synthesis of anti-viral pharmaceutical compounds.